The present invention is generally drawn to a fuel cell construction for optimizing fuel cell performance and achieving high fuel cell system efficiency and more particularly to a staged fuel cell structure for achieving same.
Fuel cells are electrochemical devices that convert the energy of a chemical reaction directly into electrical energy. The basic physical structure of a single fuel cell includes electrodes (an anode and a cathode) with an electrolyte located there between in contact with the electrodes. To produce electrochemical reaction at the electrode, a fuel stream and a oxidant stream are supplied to the anode and cathode, respectively. The fuel cell electrochemically converts a portion of the chemical energy of the fuel in the fuel stream to electricity, while the remaining amount of the chemical energy is released as heat. A stack of individual fuel cells is preferably connected in electrical series to generate a useful additive voltage.
The type of electrolyte used in a fuel cell is generally used to classify the fuel cell and is also determinative of certain fuel cell operating characteristics, such as operating temperature. Present classes of fuel cells include the Polymer Electrolyte Fuel Cell (PEFC), the Alkaline Fuel Cell (AFC), the Phosphoric Acid Fuel Cell (PAFC), the Molten Carbonate Fuel Cell (MCFC), and the Solid Oxide Fuel Cell (SOFC).
Ideally, fuel cell performance is expected to depend only on the fuel composition and the amount of fuel consumed at the anode side. However, typical voltage-current and power characteristics of operating fuel cells show a performance drop due to many resistances, including the fuel utilization resistance. This utilization resistance is primarily caused by the driving force variation (across the electrode-electrolyte assembly), which is itself due to a fuel composition gradient over the anode surface.
In fuel cell literature, various designs of anode-electrolyte-cathode and associated flow passages are available for constructing multi-layer fuel cell stacks. The most common configurations are the planar and tubular assemblies. In either case, the fuel and oxidant (e.g., air) flow past the surface of the anode and cathode placed opposite the electrolyte, respectively, so that the anode surface is in direct contact with the fuel and the cathode surface is in direct contact with air. The flow passages are connected to the inlet and outlet manifolds on both the anode and cathode sides.
In all fuel cells, the fuel composition decreases due to electrochemical reactions as the fuel passes across the anode from the inlet to the outlet. This gives rise to species concentration gradients, which are mainly responsible for uneven fuel utilization and unwanted temperature gradients on the anode surface. The cell voltages drop to adjust to the lowest electrode potential for the depleted species compositions at the exit of the anode and cathode sides.
Referring now to the drawings generally and FIG. 1 in particular, a known fuel cell assembly (10) is shown. The fuel (4) and oxidant (6), preferably air, flow past the surface of an anode (12) and cathode (14) placed on opposite sides of an electrolyte (not visible) so that the anode surface (12) is in direct contact with the flow of fuel (4) and the cathode surface (14) is in direct contact with flow of air (6). The flow passages are fluidically connected to known inlet and outlet manifolds (not shown) on both the anode (12) and cathode (14). The problems associated with this type of construction have been described above.
Accordingly, staging of fuel cells is one known way to help alleviate this problem. U.S. Pat. No. 6,033,794 “Multi-stage Fuel Cell System Method and Apparatus” discloses a fuel cell system consisting of multiple fuel cells. The gas flow paths in the cells are connected in an externally staged, serial, flow-through arrangement. This arrangement has a series of higher temperature fuel cells which utilize the increased temperature of the fuel as it exits each consecutive fuel cell in order to improve fuel cell efficiency.
Notably, no known staging of the inlet fuel to one individual fuel cell exists, although such inlet staging could provide better utilization of the fuel, a more even temperature distribution, and, generally, a more efficient fuel cell. Thus, inlet staging to a single fuel cell would be welcome by the industry, as this single cell inlet staging would permit enhanced performance of both individual cells, as well as entire stacks.